The Australian guard scored a season-high 25 points and added five rebounds, five assists and two steals in their 150-117 home victory.
It was the team's largest total since moving to Oklahoma City from Seattle before the 2008-09 season.
And they did it without regular go-to man Shai Gilgeous-Alexander (illness), who ranks among the league's leaders with 30.8 points per game.
Instead it was Giddey's wizardry that took centre stage, the Australian scoring 17 third-quarter points and - thanks to three-of-five three pointers - improving his long-range shooting to 34 per cent this season.
That's almost 10 per cent better than in his rookie campaign.
"It felt contagious from the jump, we shot the lights out of it," the 20-year-old said.
"We flowed, moved the ball well, got good looks and made a lot of them.
"If we continue to play like that ... obviously we're not always going to shoot like that, but the principles of getting those looks and defend the way we did, will help us."
The Celtics allowed 40 points in the second quarter and 48 in the third, setting up their first loss of more than 16 this season and the Thunder's first win by more than that same margin.
Boston (26-12) have gone 5-5 in their last 10 games and can be caught by Brooklyn (25-12) if they notch a 13th-straight win against Chicago on Thursday (AEDT).
Meanwhile the Thunder (16-21) remain 12th in the western conference and three wins outside the post-season play-in zone.
Giannis Antetokounmpo scored a career-high 55 points in a return from a one-game absence and Milwaukee defended home court against Washington 123-113, snapping the Wizards' five-game winning streak.
Utah lost their fifth-straight game by five points or less in heartbreaking fashion, 117-115 to Sacramento.
Down by two with less than a second to play, Lauri Markkanen caught the long in-bound pass and watched his desperate three-point floater sail in.
But replays showed he had released the ball after time expired.
De'Aaron Fox's 37 points - 22 of those in the final quarter, including the match-winner - powered the Kings.